A wheel of an automobile is rotatably supported to a suspension device by a rolling bearing unit such as a double row angular rolling bearding unit. To secure a driving stability of the automobile, a vehicle driving stabilizing device such as an antilock break system (ABS), a traction control system (TCS), and an electronic vehicle stability control system (ESC), as described in Non-Patent Document 1 has been used. To control such various kinds of vehicle driving stabilizing device, a signal representing a rotating speed, acceleration applied to a vehicle body in each direction, or the like is necessary. To more precisely control them, it may be preferable to know magnitude of a load (e.g., one or both of a radial load and an axial load) applied through the wheel to the rolling bearing unit.
In consideration of such circumstances, in Patent Document 1, there is disclosed an invention relating to a rolling bearing unit including a state measuring device for measuring a radial load or an axial load applied to the rolling bearing unit on the basis of revolution speeds of a pair of rows of balls constituting the rolling bearing unit that is a double row angular rolling bearding unit. In the rolling bearing unit including the state measuring device described in Patent Document 1, the revolution speeds of both rows of balls are obtained as revolution speeds of a pair of retainers that retain the balls, and the radial load and the axial load are calculated on the basis of the revolution speeds of both rows of the balls. In case of such a conventional configuration, delicate shift may occur between the revolution speeds of both rows of the balls and the revolution speeds of both retainers, because of a gap inevitably existing between rolling surfaces of the balls and inner surfaces of pockets of both retainers. For this reason, to precisely obtain the radial load and the axial road, there is room for improvement.
Although not disclosed, a rolling bearing unit including a state measuring device using a special encoder was invented (e.g., Japanese Patent Application No. 2005-147642) as a structure capable of preventing measurement precision from deteriorating caused by the aforementioned inevitable shift, and it is under development. FIGS. 10 to 12 illustrate an example of such a rolling bearing unit including a state measuring device using a special encoder. In the previously invented rolling bearing unit including a state measuring device, a hub 2 serving as a rotation raceway ring rotating together with a wheel supported and fixed at the time of use is rotatably supported through a plurality of rolling elements 3 and 3, inside an outer ring 1 serving as a stationary raceway ring that does not rotate even at the time of use. Precompression is applied to the rolling elements 3 and 3 with contact angles opposite to each other (in the figure, back surface combination type). In the shown example, a ball is used as the rolling element 3. However, in case of an automobile bearing unit, a tapered roller may be used instead of the ball.
In an inner end of the hub 2 (“inner” in the axial direction means a width-direction center portion of a vehicle in state of mounting on the vehicle, which is the right side in FIGS. 1, 3, 5, 6, 7, 8, and 10. On the contrary, “outer” in the axial direction means a width-direction outside of the vehicle in the state of mounting on the vehicle, which is the left side in FIGS. 1, 3, 5, 7, 8, and 10. The same is applied to the whole specification.), a cylindrical encoder 4 is fixed to be concentric with the hub 2. In a cylindrical cover 5 having a bottom to close the inner end opening of the outer ring 1, a pair of sensors 6a1 and 6a2 are supported, and detectors for both sensors 6a1 and 6a2 are close to and opposed to an outer circumferential surface that is a detection surface of the encoder 4.
The encoder 4 is made of a magnetic metal sheet. A front half portion (axial inner half portion) of the outer circumferential surface serving as the detection surface of the encoder 4, through-holes 7 and 7 (first characteristic part) and column portions 8 and 8 (second characteristic part) are alternated in a circumferential direction and are disposed at equal distances. Boundaries between the through-holes 7 and 7 and the column portions 8 and 8 are inclined to the axial direction of the encoder 4 by the same angle, and inclined directions to the axial direction are reverse with a boundary that is the axial center of the encoder 4. Accordingly, the axial centers of the through-holes 7 and 7 and the column portions 8 and 8 have a “^” shape (or “<” shape) most protruding in the circumferential direction. In the axial outer half portion and the axial inner half portion having the different inclined directions of the boundaries on the detection surface, the axial outer half portion is a first characteristic change portion 9 and the axial inner half portion is a second characteristic change portion 10. As shown, the through-holes constituting both characteristic change portions 9 and 10 may be formed continuously to each other, and may be formed independently from each other. Even through detection precision is lower, only the boundary of any one characteristic change portion of both characteristic change portions 9 and 10 may be inclined to the axial direction, and the boundary of the other characteristic change portion may be made parallel to the axial direction.
Each of the pair of sensors 6a1 and 6a2 includes a permanent magnet and magnetic detection elements such as a hall IC, a hall element, an MR element, and a GMR element constituting a detection portion. Both sensors 6a1 and 6a2 are supported and fixed in a cover 5, the detection portion of one sensor 6a1 is close to and opposed to the first characteristic change portion 9, and the detection portion of the other sensor 6a2 is close to and opposed to the second characteristic change portion 10. Both sensors 6a1 and 6a2 are opposed to both characteristic change portions 9 and 10 at the same position relative to the circumferential direction of the encoder 4. In state where an axial load is not applied between the outer ring 1 and the hub 2, positions of the members are restricted so that the most protruding portion (portion at which the inclined direction of the boundary changes) in the circumferential direction at the axial center of the through-holes 7 and 7 and the column portions 8 and 8 is located at the center between both sensors 6a1 and 6a2.
In case of the rolling bearing unit including a state measuring device configured as described above, when an axial load is applied between the outer ring 1 and the hub 2 (the outer ring 1 and the hub 2 have a relative displacement in the axial direction), phases are shifted from each other in which output signals of both sensors 6a1 and 6a2 change. That is, in a neutral state where an axial load is not applied between the outer ring 1 and the hub 2, the detection portions of both sensors 6a1 and 6a2 are opposed to solid lines A and A shown in FIG. 12A, that is, to a portion which is shifted as much as the same distance in the axial direction from the most protruding portion. Accordingly, the phases of the output signals of both sensors 6a1 and 6a2 coincide with each other, as shown in FIG. 12C.
On the contrary, when a downward axial load in FIG. 12A is applied to the hub 2 to which the encoder 4 is fixed, the detection portions of both sensors 6a1 and 6a2 are opposed to broken lines B-B shown in FIG. 12A, that is, to a portion in which shafts in the axial direction from the most protruding portion are different from each other. In this state, the phases of the output signals of both sensors 6a1 and 6a2 are shifted from each other as shown in FIG. 12B. When an upward axial load is applied to the hub 2 to which the encoder 4 is fixed in FIG. 12A, the detection portions of both sensors 6a1 and 6a2 are opposed to dashed dotted lines C-C as shown in FIG. 12A, that is, to a portion in which shifts in the axial direction from the most protruding portion are reversely different from each other. In this state, the phases of the output signals of both sensors 6a1 and 6a2 are shifted from each other as shown in FIG. 12D.
As described above, in case of the previously invented structure, the phases of the output signals of both sensors 6a1 and 6a2 are shifted from each other in a direction along a direction (direction of a relative displacement of the outer ring 1 and the hub 2 in the axial direction) in which an axial load is applied between the outer ring 1 and the hub 2. The phases of the output signals of both sensor 6a1 and 6a2 are further shifted from each other by the axial load (relative displacement) as the axial load (relative displacement) gets larger. Accordingly, when the shifts in the phases of the output signals of both sensors 6a1 and 6a2 exist, it is possible to acquire a direction and a magnitude of the relative displacement of the outer ring 1 and the hub 2 in the axial direction and to acquire a direction and a magnitude of the axial load applied between the outer ring 1 and the hub 2, on the basis of a direction and a magnitude of the shifts. The relative displacement and the load are calculated on the basis of difference in the phases of the output signals of both sensors 6a1 and 6a2 by an operator 13. For this reason, relation between the difference in phase and the axial relative displacement and the load, which are previously examined by theoretical calculation or experiment, are input to the operator 13 in a form such as a formula or a map.
In case of the previously invented structure described above, the encoder is made of a metal plate, the first characteristic portion formed on the detection surface of the encoder is formed of the through-holes, and the second characteristic portion is formed of the column portion. In addition, the encoder may be made of a permanent magnet, the first characteristic portion formed on the detection surface of the encoder may be a portion magnetized to an N pole, and the second characteristic portion may be a portion magnetized to an S pole. With such a configuration, it is not necessary to mount a permanent magnet on a pair of sensors because the encoder is made of a permanent magnet. The encoder may be formed in a wheel shape, an axial side of the encoder may be used as the detection surface, and detection portions of the pair of sensors may be opposed to the detection surface in state where the detection portions are shifted diametrically. In this case, it is possible to acquire a displacement of the outer ring 1 and the hub 2 in the radial direction, and further, to acquire a radial load applied between the outer ring 1 and the hub 2.
In controlling the aforementioned vehicle driving stabilizing devices such as ABS, TCS, and ESC, it is possible to perform a higher precise control in case where both loads (displacement) of the aforementioned axial load (displacement) and radial load (displacement) are used as control information, as compared with case where any one load (displacement) thereof is used as control information. When moment (inclination between center axes of the outer ring 1 and the hub 2) applied between the outer ring 1 and the hub 2 is used as control information in addition to both loads (displacement), it is possible to a higher precise control. For this reason, it is preferable to use a structure capable of measuring three kinds of states such as the axial load (displacement), the radial load (displacement), and the moment (inclination).
As a method for realizing such a structure, for example, there is a method in which three state measuring devices are mounted on a rolling bearing unit, such as a state measuring device including an encoder for measuring the axial load (displacement), a state measuring device including an encoder for measuring the radial load (displacement), and a state measuring device including an encoder for measuring the moment (inclination). However, when the method is employed in which the state measuring devices each including one encoder are mounted on the rolling bearing unit as many as the same number (3) as kinds of states to be measured, cost of the used state measuring devices increases and a space for mounting all the state measuring devices may not be secured according to any rolling bearing unit (e.g., a rolling bearing unit for a small-size vehicle).
Meanwhile, as an invention capable of solving such a problem to some extent, in Japanese Patent application No. 2005-147642, there is described a state measuring device capable of measuring two kinds of states of the axial load (displacement) and the moment (inclination), using one encoder. Although not disclosed, in Japanese Patent Application No. 2006-115302, there is described a state measuring device capable of measuring two kinds of states of the axial load (displacement) and the radial displacement, using one encoder. When these state measuring devices are used (two kinds among the three kinds are measured by the previously invented devices), the number of (state measuring devices including) encoders used to measure the three kinds of states can be reduced from three to two. For this reason, the aforementioned problem can be solved as much as that. However, in order to further sufficiently solve the problem, it is desired to realize a state measuring device capable of measuring the three kinds of states, using one encoder.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-31063.
Non-Patent Document 1: “Best Car Supplement Volume Entitled Red Badge Series/Book Presenting Automotive Latest Mechanisms” written by Motoo AOYAMA, p. 138 and 139, p. 146 to 149, Sansuisha Co., Ltd./Kodansha Ltd., Dec. 20, 2001.